This invention relates to means of stimulating electrically excitable tissue, and more particularly relates to means for adjusting the locus at which action potentials are induced in such tissue.
Two major practical problems reduce the efficacy of epidural spinal cord stimulation (SCS) for pain control. One is the difficulty of directing the stimulation-induced paresthesia to the desired body part and the other is the problem of disagreeable sensations or motor responses to the stimulation, which reduce the comfortable amplitude range of the stimulation. It is generally agreed that in SCS, for chronic pain, paresthesia should cover the whole pain region. With present stimulation methods and equipment, only highly skilled and experienced practitioners are able to position a stimulation lead in such a way that the desired overlap is reached and desired results are obtained over time with minimal side effects. It requires much time and effort to focus the stimulation on the desired body region during surgery and, using pulses with single value cathodes, it is difficult to redirect it afterwards, even though some readjustments can be made by selecting a different contact combination, pulse rate, pulse width or voltage.
Redirecting paresthesia after surgery is highly desirable. Even if paresthesia covers the pain area perfectly during surgery, the required paresthesia pattern often changes later due to lead migration, or histological changes (such as the growth of connective tissue around the stimulation electrode) or disease progression. The problem of lead placement has been addressed by U.S. Pat. No. 5,121,754 by the use of a lead with a deformable distal shape. These problems are not only found with SCS, but also with peripheral nerve stimulation (PNS), depth brain stimulation (DBS), cortical-stimulation and also muscle or cardiac stimulation;
The era of precise control of electrical fields for excitation of tissue by use of multiple voltages is disclosed in PCT International Publication No. WO 95/19804 (counterpart to Holsheimer et al., U.S. Pat. No. 5,501,703) (the xe2x80x9cHolsheimer referencesxe2x80x9d). The Holsheimer references describe the use of electrodes that are xe2x80x9cin-line,xe2x80x9d namely that they are disposed xe2x80x9csymmetricallyxe2x80x9d along a line. The three juxtaposed electrodes have two simultaneous voltages relative to one of them, each with its own amplitude. This approach allows xe2x80x9csteeringxe2x80x9d of the electric fields created by these electrodes. Particularly, the electrical field pattern is adjusted by varying the electrical field generated between those electrodes along that line. The locus of excitation is correspondingly varied with that variation in the electrical field pattern. For example, if a central electrode of three roughly collinear electrodes is a cathode (xe2x88x92) then the outer anodes push the areas of excitation toward the middle, and shield outer areas from excitation. As the anodal pulses are varied in amplitude, the field steers toward the outside.
However, the Holsheimer references disclose a system that requires three electrodes that are optimally spaced symmetrically along a line. It is a serious handicap during the surgical procedure to place these electrodes in the body. Qften, a lead such as a paddle is used for mounting the multiple electrodes in the optimally spaced positions. This lead is then inserted within a patient near the tissue to be excited, and electrical excitation is applied to the lead. Unfortunately, placement of a lead such as the paddle within a patient, can be difficult since the paddle can be surgically difficult to manipulate adjacent the spinal cord. Thus, it would be desirable to be able to adjust the locus of excitation in electrically excitable tissue without the use of optimally spaced electrodes.
In addition, the Holsheimer system is limited in that steering is accomplished over a linear path. It would be desirable to adjust the locus of excitation in electrically excitable tissue over a greater area.
Accordingly, a primary object of the present invention is to provide a method and apparatus for adjusting the locus of excitation in electrically excitable tissue using electrodes that do not have to be optimally spaced in a line.
In particular, an object of the present invention is to adjust areas of subthreshold excitation in order to adjust an area of superposition of such areas of subthreshold excitation. The area of superposition determines the locus of excitation of electrically excitable tissue.
Another object of the invention is to provide a method and apparatus for adjusting the locus of super threshold excitation in electrically excitable tissue using electrodes that are spaced in a two dimensional array.
Another object of the invention is to add outer anodes to a grouping of cathodal electrodes to shield areas farther out and to keep activation of tissue nearer to the cathodes.
In a principle aspect, the present invention takes the form of an apparatus and method for inducing action potentials at an adjustable locus of electrically excitable tissue. In accordance with the invention, a first pulse having a first amplitude and a first pulse width is applied to the tissue via. a first electrode adapted to be adjacent said tissue. Similarly, a second pulse having a second amplitude and a second pulse width is applied to the tissue via a second electrode adapted to be adjacent said tissue.
The application of the first pulse generates a first subthreshold potential area in said tissue, and the application of the second pulse generates a second subthreshold potential area. The first subthreshold area is determined by the first amplitude and the first pulse width of the first pulse, and the second subthreshold area is determined by the second amplitude and the second pulse width of the second pulse. A superposition of the first and second subthreshold areas results in a suprathreshold potential area of said adjustable locus where the action potentials are induced.
This embodiment of the present invention may be applied to particular advantage when adjusting the locus where the action potentials are induced. The first amplitude or the first pulse width of the first pulse can be adjusted for a corresponding adjustment of the first subthreshold area and contribute, in turn, to the volume where suprathreshold potentials are produced. Similarly, the second amplitude or the second pulse width of the second pulse can be adjusted for a corresponding adjustment of the second subthreshold area and contribute, in turn, to the volume of where suprathreshold potentials are produced. The size and location of the suprathreshold potential area can thus be controlled.
In another aspect of the present invention, a time delay between the application of the first and second pulses can be varied for a corresponding adjustment in size and location of the suprathreshold potential area. The time delay between the application of the first and second pulses can be measured from the end time of the first pulse to the begin time of the second pulse. Additionally, that delay can be measured as a difference between a first weighted average time of the first pulse and a second weighted average time of the second pulse, or between a first peak time of the first pulse and a second peak time of the second pulse.
In another aspect of the invention, simultaneous pulses of varying amplitudes are delivered to multiple electrodes (cathodes), which are arranged in a two-dimensional array. As a cross-pattern, there may be a central electrode at the center of the pattern, which is the most cathodal (negative). By having the outer four electrodes to be less cathodal (not as negative), or even fully positive (anodal), the locus of cells that have suprathreshold activation can be shifted in two dimensions. With such constraining of the fields, the amplitude can be increased, driving the locus of activation deeper into the tissue, thereby creating a third dimensional effect.
In yet another aspect of the invention, the two-dimensional array of cathodes may be surrounded by an outer ring of anodes to keep the locus of activation contained and to shield outside tissue from activation.
In still another aspect of the invention, a combination of simultaneous and delayed cathodal pulses are applied on some electrodes in an array. Each pulse creates an area of subthreshold excitation, and the combination provides a controlled locus to the threshold for the production of action potentials.
These and other features and advantages of the present invention will be better understood by considering the following detailed description of the invention which is presented with the attached drawings.